Electrochemical cells

ABSTRACT

An electrochemical cell comprises a first electrode, a second electrode, a porous separator, between the first and second electrodes, a first channel, having an inlet and an outlet, and a second channel, having an inlet and an outlet. The first channel is contiguous with the first electrode and the porous separator, and the second channel is contiguous with the second electrode and the porous separator.

PRIORITY CLAIM

This application claims priority from a provisional patent applicationentitled “Electrochemical Cells Involving Laminar Flow Induced DynamicConducting Interfaces” with reference number 60/610281, filed on Sep.15, 2004.

FIELD OF INVENTION

The present invention relates to electrochemical devices forelectrochemical energy conversion (e.g., fuel cells and batteries). Morespecifically, the present invention teaches a variety of electrochemicaldevices utilizing channels contiguous to a porous separator, gasdiffusion electrodes, and laminar flow.

BACKGROUND

Fuel cell technology shows great promise as an alternative energy sourcefor numerous applications. Several types of fuel cells have beenconstructed, including: polymer electrolyte membrane fuel cells, directmethanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells,molten carbonate fuel cells, and solid oxide fuel cells. For acomparison of several fuel cell technologies, see Los Alamos NationalLaboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power bySharon Thomas and Marcia Zalbowitz.

Although all fuel cells operate under similar principles, the physicalcomponents, chemistries, and operating temperatures of the cells varygreatly. For example, operating temperatures can vary from roomtemperature to about 1000° C. In mobile applications (for example,vehicular and/or portable microelectronic power sources), afast-starting, low weight, and low cost fuel cell capable of high powerdensity is required. To date, polymer electrolyte fuel cells (PEFCs)have been the system of choice for such applications because of theirlow operating temperatures (e.g., 60-120° C.), and inherent ability forfast start-ups.

Prior Art FIG. 1 shows a cross-sectional schematic illustration of apolymer electrolyte fuel cell 2. PEFC 2 includes a high surface areaanode 4 that acts as a conductor, an anode catalyst 6 (typicallyplatinum), a high surface area cathode 8 that acts as a conductor, acathode catalyst 10 (typically platinum), and a polymer electrolytemembrane (PEM) 12 that serves as a solid electrolyte for the cell. ThePEM 12 physically separates anode 4 and cathode 8. Fuel in the gasand/or liquid phase (typically hydrogen or an alcohol) is brought overthe anode catalyst 6 where it is oxidized to produce protons andelectrons in the case of hydrogen fuel, and protons, electrons, andcarbon dioxide in the case of an alcohol fuel. The electrons flowthrough an external circuit 16 to the cathode 8 where air, oxygen, or anaqueous oxidant (e.g., peroxide) is being constantly fed. Protonsproduced at the anode 4 selectively diffuse through PEM 12 to cathode 8,where oxygen is reduced in the presence of protons and electrons atcathode catalyst 10 to produce water. When either the fuel or theoxidant (or both) is in gaseous form a gas diffusion electrode (GDE) maybe used for the corresponding electrode. A GDE, which is availablecommercially, typically includes a porous conductor (such as carbon),allowing the gas to reach the electrode as well as the catalyst. Often,the catalyst is bound to the PEM, which is in contact with the GDE.Examples of GDEs and fuel cell systems which include GDEs, are describein U.S. Patent Application Publication 2004/0209154, published 21 Oct.2004, to Ren et al.

Numerous liquid fuels are available. Notwithstanding, methanol hasemerged as being of particular importance for use in fuel cellapplications. Prior Art FIG. 2 shows a cross-sectional schematicillustration of a direct methanol fuel cell (DMFC) 18. Theelectrochemical half reactions for a DMFC are as follows:Anode: CH₃OH+H₂O→CO₂+6 H⁺+6 e⁻Cathode: 3/2 O₂+6 H⁺+6 e⁻→3 H₂OCell Reaction: CH₃OH+3/2 O₂→CO₂+2 H₂O

As shown in FIG. 2, the cell utilizes methanol fuel directly, and doesnot require a preliminary reformation step. DMFCs are of increasinginterest for producing electrical energy in mobile power (low energy)applications. However, at present, several fundamental limitations haveimpeded the development and commercialization of DMFCs.

One of the major problems associated with conventional DMFCs is that thematerial used to separate the liquid fuel feed (i.e., methanol) from thegaseous oxidant feed (i.e., oxygen) is typically a stationary polymerelectrolyte membrane (PEM) of the type developed for use with gaseoushydrogen fuel feeds. These PEMs, in general, are not fully impermeableto methanol or other dissolved fuels. As a result, an undesirableoccurrence known as “methanol crossover” takes place, whereby methanoltravels from the anode to the cathode catalyst through the membranewhere it reacts directly in the presence of oxygen to produce heat,water, carbon dioxide and no useable electric current. In addition tobeing an inherent waste of fuel, methanol crossover also causesdepolarization losses (mixed potential) at the cathode and, in general,leads to decreased cell performance.

A new type of fuel cell, a laminar flow fuel cell (hereinafter “LFFC”)uses the laminar flow properties of liquid streams to limit the mixingor crossover between fuel and oxidant streams and to create a dynamicconducting interface (hereinafter “induced dynamic conducting interface”or “IDCI”), which can in some LFFC designs wholly replaces thestationary PEMs or salt bridges of conventional electrochemical devices.The IDCI can maintain concentration gradients over considerable flowdistances and residence times depending on the dissolved species and thedimensions of the flow channel. This type of fuel cell is described inU.S. Pat. No. 6,713,206, issued 30 Mar. 2004 to Markoski et al.

A fuel cell 20 embodying features of this type of flow cell design isshown in Prior Art FIG. 3. In this design, both the fuel input 22 (e.g.an aqueous solution containing MeOH and a proton electrolyte source) andthe oxidant input 24 (e.g., a solution containing dissolved oxygen,potassium permanganate or hydrogen peroxide and a proton electrolytesource) are in liquid form. By pumping the two solutions into themicrochannel 26, parallel laminar flow induces a dynamic protonconducting interface 28 that is maintained during fluid flow. If theflow rates of the two fluids are kept constant and the electrodes areproperly deposited on the bottom and/or top surfaces of the channel, theIDCI is established between anode 30 and cathode 32 and thus completesthe electric circuit while keeping the fuel and oxidant streams fromtouching the wrong electrode. In this particular LFFC design theelectrodes are in a side-by-side configuration.

A fuel cell may have a face to face LFFC design. In this design, boththe fuel input (e.g. an aqueous solution containing a fuel and a protonelectrolyte source) and the oxidant input. (e.g., a solution containingdissolved oxygen, potassium permanganate or hydrogen peroxide, and aproton electrolyte source) are in liquid form. By pumping the twosolutions into the microchannel, parallel laminar flow induces a dynamicconducting interface that is maintained during fluid flow between theanode and the cathode and thus completes the electric circuit whilekeeping the flowing fuel and oxidant streams from touching the wrongelectrode. If the fuel and oxidant flow rates are the same, the IDCIwill be established directly in the middle of the flow channel. The faceto face LFFC offers significant operational flexibility as a result ofthe ability to position the IDCI flexibly between the electrodes withoutexperiencing significant cross-over effects and offers significantperformance capabilities due the potential for lower internal cellresistance because of the relatively short and uniform electrode toelectrode distances not afforded with the side by side design. Withinthis face to face design there exist a number of potential flowgeometries that could be used. LFFCs with identical cross-sectionalareas, but having different channel widths and heights andelectrode-electrode distances are possible, however the best choice indesign has the lowest electrode to electrode distance and the highestactive area to volume ratio. In general a relatively short height andbroad width is preferred and will provide the best overall performanceunder cell operation when positioned orthogonal to the gravitationalfield. However, if the optimized face to face LFFCs are tilted or joltedthe streams can flip or twist causing the fuel and oxidant to come incontact with the wrong electrode, leading to cross-over, catastrophicfailure, and/or cell reversal until the stable fluid flow can bere-established. These phenomena severely limit the applicability andusefulness of LFFCs. An improvement is needed to the optimal face toface design that still utilizes all of its performance advantages whilestabilizing the fluid flows under all gravitational orientations, andshock-like conditions as well as allowing the streams to be split andrecycled.

SUMMARY

The present invention teaches a variety of electrochemical devices forelectrochemical energy conversion. In one embodiment, the presentinvention teaches an electrochemical cell, comprising a first electrode,a second electrode, a porous separator, between the first and secondelectrodes, a first channel, having an inlet and an outlet, and a secondchannel, having an inlet and an outlet. The first channel is contiguouswith the first electrode and the porous separator, and the secondchannel is contiguous with the second electrode and the porousseparator.

In an alternate embodiment, the present invention teaches a method ofgenerating electricity, comprising flowing a first liquid through afirst channel; and flowing a second liquid through a second channel. Thefirst channel is contiguous with a first electrode and a porousseparator, the second channel is contiguous with a second electrode andthe porous separator, the first liquid is in contact with the firstelectrode and the porous separator, the second liquid is in contact withthe second electrode and the porous separator, and complementary halfcell reactions take place at the first and second electrodes.

In an alternate embodiment, the present invention teaches anelectrochemical cell, comprising a first electrode, a second electrode,a first channel, contiguous with the first and second electrodes. Thefirst electrode is a gas diffusion electrode, such that when a firstliquid flows through the channel in contact with the first electrode anda second liquid flows through the channel in contact with the secondelectrode, laminar flow is established in both the first and secondliquids.

In an alternate embodiment, the present invention teaches a method ofgenerating electricity, comprising flowing a first liquid through achannel; and flowing a second liquid through the channel. The channel iscontiguous with a first electrode and a second electrode, the firstliquid is in contact with the first electrode, the second liquid is incontact with the second electrode, the first electrode is a gasdiffusion electrode, and complementary half cell reactions take place atthe first and second electrodes.

In a fifth aspect, the present invention is an electrochemical cell,comprising a first electrode, and a second electrode. The firstelectrode is a gas diffusion electrode, and ions travel from the firstelectrode to the second electrode without traversing a membrane.

BRIEF DESCRIPTION OF THE DRAWINGS

Prior Art FIG. 1 shows a cross-sectional schematic illustration of apolymer electrolyte fuel cell.

Prior Art FIG. 2 shows a cross-sectional schematic illustration of adirect methanol fuel cell.

Prior Art FIG. 3 shows a schematic illustration of a direct methanolfuel cell containing a laminar flow induced dynamic interface in a sideby side electrode configuration

FIG. 3A shows a schematic illustration of a direct liquid fuel cellcontaining a laminar flow induced dynamic interface in a face to faceelectrode configuration.

FIG. 4 illustrates an embodiment of a fuel cell including a porousseparator.

FIGS. 5 and 5A illustrate an embodiment of a fuel cell including aporous separator.

FIGS. 6 and 6A illustrate an embodiment of a fuel cell using gaseousoxygen.

FIG. 7 illustrates an embodiment of a system including a fuel cell.

FIG. 8 is a graph of transport limited load curves for individual LFFCswith recycle capability.

FIG. 9 is a graph of cell potential versus current density for a 1×5LFFC array.

FIG. 10 is a graph of polarization curves for a LFFC operated at roomtemperature at different fuel concentrations.

FIG. 11 is a graph comparing performance of a commercially availableDMFC and a 1×5 LFFC array, both operated at 50° C.

DETAILED DESCRIPTION

Among other things, the present invention teaches that inclusion of aporous separator (also referred to as a porous plate) between theflowing streams of a laminar flow fuel cell (hereinafter “LFFC”) allowsthe stream position to be stabilized, defined, and maintained under mostconditions. This stabilization also provides a reliable mechanism sothat individual streams can be separated and recycled. The porousseparator does not significantly impede ion conduction between thestreams. In addition, inclusion of a porous separator reduces fuelcrossover, even allowing for turbulent flow and even two-phasegas/liquid plug flow within the individual streams. The presentinvention also teaches that inclusion of an electrolyte stream, betweenthe fuel stream and the cathode, or between the oxidant stream and theanode, allows for incorporation of a gas diffusion electrode as thecathode or anode, respectively.

Throughout this description and in the appended claims, the phrase“electrochemical cell” is to be understood in the very general sense ofany seat of electromotive force (as defined in Fundamentals of Physics,Extended Third Edition by David Halliday and Robert Resnick, John Wiley& Sons, New York, 1988, 662 ff.). The phrase “electrochemical cell”refers to both galvanic (i.e., voltaic) cells and electrolytic cells,and subsumes the definitions of batteries, fuel cells, photocells(photovoltaic cells), thermopiles, electric generators, electrostaticgenerators, solar cells, and the like. In addition, throughout thisdescription and in the appended claims, the phrase “complementary halfcell reactions” is to be understood in the very general sense ofoxidation and reduction reactions occurring in an electrochemical cell.

FIG. 4 illustrates an embodiment of a fuel cell including a porousseparator. In one embodiment of the present invention, the fuel cellincludes a track etch separator 1625 (the porous separator), allowingfor separation of the fuel stream 1670 and oxidant stream 1660 flowinginto the fuel cell. The fuel stream 1670 flows past anode 1620 and theoxidant stream 1660 flows past cathode 1610, allowing for diffusion ofions between the streams (especially across diffusion zone 1640) anddepletion of fuel and oxidant (especially along depletion zones 1650).Depleted oxidant stream 1680 and depleted fuel stream 1690 then exit thefuel cell.

The porous separator separates different streams, allowing them to beeasily directed in different direction, and is particularly useful forkeeping oxidant, fuel, and/or electrolyte streams separate forsubsequent recycling. The porous separator achieves this goal withoutinterfering significantly with ion transport between the streams. Theporous separator is hydrophilic, so the fluid within the streams isdrawn into the pores by capillary action, and therefore the two streamsof fluid on either side of the separator are in contact, allowing iontransport between the two streams. Furthermore, when the pores are smalland the total area of the pores is a small percentage of the total areaof the porous separator, mass transfer of fluid from one stream to theother is very small, even if there is a significant difference inpressure between the streams; this reduces fuel crossover beyond thealready low fuel crossover of LFFCs. Finally, gas cannot easily passthrough the porous separator, since a large overpressure of gas isnecessary to displace fluid from the pores.

Although the thickness of the porous separator, diameter of the poresize, pore density and porosity can be any measurement suitable forimplementation, an example of some possible ranges is useful. Inalternate embodiments, for example, the porous separator can have athickness of 0.5 to 1000 microns, 1 to 100 microns, or 6 to 25 microns.Additionally, in alternate embodiments, the average diameter of thepores (pore size) of the porous separator can be, for example, 1 nm to100 microns, 5 nm to 5 microns, or 10 to 100 nm. The diameter of anyindividual pore is the diameter of a circle having the same area as thepore, as directly observed under a microscope. Further, in alternateembodiments, the pore density can be, for example, 10⁴ to 10¹²pores/cm², 10⁶ to 10¹¹ pores/cm², or 10⁷ to 10¹⁰ pores/cm². Pore densitycan be determined by counting the number of pores in a sample portion ofthe porous separator, as directly observed under a microscope.Additionally, in alternate embodiments, porosity, which is the surfacearea of all the pores divided by the total surface area of the porousseparator, can be, for example, 0.01 to 70%, 0.1 to 50%, or 1 to 25%.The porosity may be determined from the average pore diameter, the poredensity, and the area of the porous separator:porosity=π(density)(average diameter)/(area of separator).

The porous separator can be made of any suitable material, such as amaterial which is inert to the fluids it will come into contact withduring operation within the electrochemical cell, at the temperature atwhich it will operate. For example, metals, ceramics, semiconductorsincluding silicon, organic materials including polymers, plastics andcombinations, as well as natural materials and composites, may be used.Polymers, plastics and combinations are particularly preferred.Especially preferred are commercially available track etched filters,which are polymers films that have been bombarded with ions, and thenchemically etched to form thru-pores along the track traveled by theions. A summary of the physical properties of commercially availablepolycarbonate track etch materials is listed in the table below. porepore thick- minimum typical water size density ness weight water bubbleflow rate (um) (pores/cm²⁾ (um) (mg/cm²⁾ point (psi) (ml/min/cm²)^(A) 22 × 10⁶ 10 1.0 0.55 350 1 2 × 10⁷ 11 1.0 0.76 250 0.8 3 × 10⁷ 9 1.0 15215 0.4 1 × 10⁸ 10 1.0 36 70 0.2 3 × 10⁸ 10 1.0 70 20 0.1 3 × 10⁸ 6 0.695 4 0.08 6 × 10⁸ 6 0.6 >100 2 0.05 6 × 10⁸ 6 0.6 >100 0.7 0.03 6 × 10⁸6 0.6 >100 0.15 0.015 6 × 10⁸ 6 0.6 >100 <0.1^(A)10 psi pressure drop

FIGS. 5 and 5A illustrate an embodiment of a fuel cell including aporous separator. A layer or film 1745 (for example, Kapton or etchedglass) and a second film 1755 (for example, Kapton, etched glass orplatinum) are placed between the electrodes with catalyst 1740 (forexample, platinum foils, or a conductor such as graphite or highly dopedsilicon with a catalyst on the surface). Between the two films 1745 and1755 is porous separator 1775, which together help define the oxidantstream channel 1760 and fuel stream channel 1750. Optionally, a filmpermeable to ions (such as NAFION) may be used as the surface of theelectrode associated with the fuel stream 1750. The porous separator1775 defines the channels for the two streams 1750 and 1760, and stillallows for ion transport through the pores. Contact pads (notillustrated), such as gold, may be formed on the outside of theelectrodes to aid in electrically connecting the electrochemical cell toother devices. Also shown in FIG. 5A is the catalyst layer 1735.

FIGS. 6 and 6A illustrates an embodiment of an electrochemical cellusing a gaseous oxidant, such as O₂ or air. The fuel cell includes anoptional porous separator 1825, allowing for separation of the fuel 1870and electrolyte 1835 flowing into the fuel cell. Electrolyte 1835 flowsalong an optional film permeable to ions 1845, or when the filmpermeable to ions is absent, along the cathode 1810, which is a GDE.Gaseous oxidant 1860 flows along the GDE 1810 which receives oxygenmolecules. In some embodiments, gaseous oxidant 1860 is provided at apressure such that the same type of laminar flow may be observed betweengaseous oxidant 1860 and electrolyte 1835 as is observed in the fuel andelectrolyte streams along porous separator 1825. While pressure drop-offvaries differently in a channel for liquids and gases, maintaining anadequate pressure where the depleted oxidant 1880 exits will result insufficient pressure of gaseous oxidant 1860 to cause essentially one-waydiffusion of oxidant through the GDE (cathode) 1810. Thus, under suchconditions, the electrolyte 1835 may only minimally diffuse into thegaseous oxidant 1835 creating a three-phase interface within thecatalyst layer. When pure oxygen is used as the gaseous oxidant 1860, nodepleted oxidant is formed and therefore an exit is not necessary; thechannel through which the oxidant flows may be closed off or having adead end near the bottom of the cathode 1810. Also shown in FIG. 6A arethe electrodes with catalyst 1840 (for example, a graphite plate withcatalyst), a layer or film 1845 (for example, Kapton), and anotherelectrode 1830 (for example, graphite).

With fuel 1870 flowing past anode 1820 and electrolyte 1835 incombination with gaseous oxidant 1860 flowing past cathode 1810, ionsdiffuse across the porous separator (or in the absence of a porousseparator, ions diffuse across the IDCI formed at the interface betweenthe electrolyte stream 1835 and fuel stream 1870), especially indiffusion zone 1840 and ions are depleted along depletion zones 1850.Depleted gaseous oxidant 1880, electrolyte 1835 and depleted fuel 1890then exit the fuel cell. As illustrated, optionally, the electrolyte1835 may be recycled and returned to the fuel cell, and any fuelremaining in the depleted fuel 1890 may also be recycled and returned tothe fuel cell.

GDEs, many of which are commercially available, include a porousconductor and, preferably a catalyst, so that a complementary half cellreaction may take place on the conductor, between gaseous oxidant andions in a liquid (for example, H⁺ ions in the electrolyte). Typically, aporous hydrophobic layer is present on the GDE, on which the catalyst ispresent. Preferably, the GDE is a porous conductor with catalyst on theconductor, and has a hydrophilic surface, allowing liquid to wet theporous conductor and water produced at the GDE to spread out along thesurface of the GDE and evaporate into the gaseous oxidant or flow intothe circulating electrolyte. Any coating or layers present on the sideof the GDE facing the electrolyte must allow for the conduction of ionsto the catalyst layer without allowing significant liquid breakthroughor flooding into the gas flow stream. For example, the GDE may include aporous carbon substrate, such as teflonized (0-50%) Torray paper of50-250 micron thickness (a porous conductor available from SGL CarbonAG, Wiesbaden, Germany) onto which is bonded the catalyzed (e.g. 4mg/cm² Pt black) surface of a film permeable to ions or porous layer,such as NAFION 112 or expanded polyethylene, having a total thickness of50 microns or less. The circulating electrolyte may be, for example,0.5-2.0 M sulfuric acid. Unlike a NAFION film used in a PEFC, the filmused with a GDE in the present invention typically will not havecatalyst on both sides of the film; rather catalyst will only be presenton one side of the film.

Although the current density produced by the fuel cells can vary widelydepending on a variety of factors, an example of some possible ranges isuseful. In one embodiment of the present invention, the fuels cells canproduce, for example, at least 50 mA/cm². In an alternate embodiment,the fuels cells can produce, for example, at least 400 mA/cm². Further,in other embodiments, the fuel cells can produce, for example, at least1000 mA/cm², including 100-1000 mA/cm², 200-800 mA/cm², and 400-600mA/cm².

Various fuel cells have been discussed. Each fuel cell is likely to beincorporated into a module or component along with support technology toprovide a power supply. As a result, it may be useful to provide a powersupply implementation using such fuel cells.

FIG. 7 illustrates an embodiment of a power system including a fuelcell. The power system uses a fuel cell and supporting components toproduce power. Those supporting components include fuel andelectrolytes, a pump and a blower, a power regulator, a battery powersupply and various control components. For example, a power systemincludes fuel cell stack 1910, which may be a stack of fuel cells suchas those of the present invention. Coupled to fuel cell stack 1910 isdual pump 1920, which provides fuel from fuel mixing chamber 1950 andelectrolyte from electrolyte reservoir 1940. Dual pump 1920 may bereplaced with two single pumps in alternate embodiments. Mixing chamber1950 receives depleted fuel from fuel cell stack 1910 (through itsoutput) and fuel from fuel reservoir 1930 through control valve 1960.Similarly, electrolyte reservoir 1940 receives electrolyte fluid fromfuel cell stack 1910 and may also receive depleted oxidant (e.g. airdepleted of oxygen) from fuel cell stack 1910. The depleted oxidant mayalso enter the electrolyte reservoir 1940 and then exit. As theelectrolyte is preferably not depleted by the process of the fuel cellstack 1910, it should not need to be refilled often. Fuel reservoir 1930may be filled as required to provide fuel to the system. To keep fuel atdesirable levels in both mixing chamber 1950 and fuel reservoir 1930,carbon dioxide may fill an empty mixing chamber 1950, and be forced intofuel reservoir 1930 as fuel fills mixing chamber 1950. Excess carbondioxide may be bled out of the system.

To provide gaseous oxygen (from a dedicated oxygen supply or fromambient air for example), blower 1970 blows gaseous oxygen into fuelcell stack 1910. Blower 1970, pump 1920 and control valve 1960 may allbe powered by DC-DC converter 1980, which in turn draws power primarilyfrom fuel cell stack 1910. Converter 1980 potentially operates as avoltage or power regulator to provide an 18 W output in someembodiments. Typically, an 18 W output may be predicated on a 20 Woutput from fuel cell 1910, for example. This allows 2 W for overhead,namely running the blower 1970, pump 1920 and control valve 1960, whichis a reasonable amount of power for such components.

Note that interruptions may occur in power supplied from fuel cell stack1910, between obvious startup delays (the fuel cells need fuel togenerate power) and occasional disruptions due to, for example, airbubbles in fuel or electrolyte. Thus, battery 1990 is provided to powerthe system at startup and provide small amounts of power in undersupplysituations. Battery 1990 may be rechargeable or non-rechargeable, andpreferably will not need replacement except at rare intervals.

The electrochemical cell technology described herein is applicable tonumerous systems including batteries, fuel cells, and photoelectriccells. It is contemplated that this technology will be especially usefulin portable and mobile fuel cell systems and other electronic devices,such as in cellular phones, laptop computers, DVD players, televisions,palm pilots, calculators, pagers, hand-held video games, remotecontrols, tape cassettes, CD players, AM and FM radios, audio recorders,video recorders, cameras, digital cameras, navigation systems,wristwatches and other electronics requiring a power supply. It is alsocontemplated that this technology will also be useful in automotive andaviation systems, including systems used in aerospace vehicles.

The following description provides some example implementationscontemplated by the present invention for conversion of chemical energyof a fuel into electricity based on the embodiments described herein.This set of examples is by no means an exhaustive set and is merelyreflective of the wide scope of applicability of the present invention.

EXAMPLE 1 Single Channel LFFC with Dissolved Oxidant

A 25 um Pt layer provided the channel height for the anode and the Ptlayer also served as the current collector for the catalyst layer above.The catalyst layer was 4.0 mg/cm² Pt/Ru catalyst bonded to the surfaceof a NAFION 117 film. A 25 um Kapton layer provided the channel heightfor the cathode and the 25 um Pt layer served as the cathode catalystand current collector. The electrode to electrode distance was 56 um andthe porous layer used to separate the anode from the cathode was a 6 umthick polycarbonate track etched layer with 100 nm pores and 6×10⁸pores/cm². This equates to approximately 2-4% porosity. 200 nm poresizes with 8-12% porosity and a film thickness of 12 um were alsoevaluated in order to optimize the track etch performance. Channeldimensions were 1.0 mm width, 50 micron height, and 30 mm length. If allof the Kapton layers, track etch layer, and current collectors were veryflat and aligned, no external leak points were observed while held underan external compression field (100-500 lbs).

For the experiments shown in FIG. 8, 1 M Methanol in 2 M H₂SO₄ was usedas the fuel and 0.1 M-0.2 M KMnO₄ in 2 M H₂SO₄ was used as the oxidant.Flow rates were varied between 0.3-0.6 mL/min. These flow rates providedapproximately 5-15 psi backpressure with these channel dimensions. Ascan be seen in FIG. 8, transport limitations were observed at lower flowrates and lower oxidant concentrations indicating that the cell wascathode limited. CO₂ bubble formation could be observed only in the fueleffluent above approximately 150 mA/cm². The presence of bubbles in thefuel effluent did not observably reduce cell performance. The absence ofa purple color from the fuel effluent also indicated little to nointernal mixing of the fuel and oxidant streams which were completelyseparated upon exiting the cell.

EXAMPLE 2 Multi-Channel LFFC with Dissolved Oxidant

An externally manifold 1×5 LFFC array was fabricated. A 25 um Kaptonspacer layer plus a 25 um Pt layer provided the channel height for theanode and the Pt layer also served as the current collector (edgecollection) for the catalyst layer above. The anode catalyst layer was4.0 mg/cm² Pt/Ru on a NAFION 117 film that was then thermally bonded(hot pressed) with a 3M thermal setting epoxy-type adhesive layer to a125 um Kapton film to provide rigidity and mechanically integrity(flatness) to the catalyst layer. A 50 um Kapton layer provided thechannel height for the cathode and the 25 um Pt layer served as thecathode catalyst and current collector. The electrode to electrodedistance was 112 um and the porous layer used to separate the anode fromthe cathode was a 12 um thick Kapton film track etched with 100 nm poresand 1×10⁹ pores/cm². This equates to approximately 8% porosity. 50, 75,and 100 nm pore sizes with 1-15% porosity in film thickness of 7, 12 and25 um were evaluated in order to optimize the track etch performance.Channel dimensions were 1.5 mm width, 112 micron height, and 30 mmlength. If all of the Kapton layers, track etch layer, and currentcollectors were very flat and aligned, no external leak points wereobserved while held under an external compression field (100-500 lbs).Near even flow distribution was also observed with these un-bondedlayers. For the experiments shown in FIG. 9, 1 M formic acid in 2 MH₂SO₄ was used as the fuel and 0.1 M KMnO₄ in 2 M H₂SO₄ was used as theoxidant. A flow rate of 2 mL/min/channel was used in all cases. Thisflow rate provided approximately 5 psi backpressure with this channelheight. As can be seen in FIG. 9, high current densities were stillachieved with multiple channels in parallel and CO₂ bubble formationcould be observed in the fuel effluent around 150 mA/cm², however notall channels provided identical load curves despite having equal flowwhich may be explained as a result of unequal catalyst distribution orcurrent collection. The presence of bubbles in the fuel effluent did notreduce cell performance. The absence of a purple color from the fueleffluent also indicated little to no internal mixing of the fuel andoxidant streams which were completely separated upon exiting the cell.

EXAMPLE 3 Multi-Channel LFFC with Internally Replenishable Oxidant

An externally manifold 1×5 LFFC array was fabricated. A catalyzedgraphite sheet (1 mm) was the anode. A 50 um Kapton layer provided thechannel height for the anode. A 50 um Kapton layer provided the channelheight for the electrolyte. The porous layer separating the anode fromthe electrolyte was composed of a 6 um thick polycarbonate track etchedlayer with 100 nm pores and 6×10⁸ pores/cm². This equates toapproximately 2-4% porosity. Liquid channel dimensions were 1.5 mmwidth, 50 micron height, and 30 mm length. The electrode to electrodedistance was 130 um. The cathode was composed of a 25 um NAFION 111bonded to a pre-catalyzed 250 um GDE with the gas porous side exposed to0.5 mm graphite gas flow channels and the NAFION side exposed to theelectrolyte. If all of the Kapton layers, track etch layer, GDE, andcurrent collectors were very flat and aligned, no external leak pointswere observed while held under an external compression field (100-500lbs). Near even fluid distribution between the channels was alsoobserved with these un-bonded layers. FIG. 10 illustrates the roomtemperature performance improvements that occurred as a result ofincreasing fuel concentration of methanol in 1.0 M sulfuric acid for thefuel stream (4 mL/min total), 1.0 M sulfuric acid for the electrolytestream (4 mL/min total), and ambient oxygen (1000 mL/min total). Theanode was 5 mg/cm² 50/50 Pt/Ru black deposited onto a graphite plate,and the cathode was 2 mg/cm² 50% Pt/C and 4 mg/cm² Pt black depositedonto a GDE. As can be seen in FIG. 10, high current densities were stillachieved with multiple channels in parallel and CO₂ bubble formationcould be observed only in the fuel effluent around and above 150 mA/cm².The presence of bubbles in the fuel effluent did not reduce cellperformance. The absence of bubbles in the electrolyte and the absenceof liquid in the gas effluent indicated little to no internal mixing ofthe fuel, electrolyte and oxidant streams which were completelyseparated upon exiting the cell. A slight performance decrease wasobserved with 12 M MeOH which was determined to be a result of increasedcell resistance and not fuel cross-over.

Elevated temperature effects on the externally manifold 1×5 LFFCdescribed above were investigated and a comparison to a commerciallyavailable DMFC (5 cm² with NAFION 117 membrane electrode assembly) underidentical operating and temperature conditions was made, except that theDMFC did not have any sulfuric acid in the fuel stream. By raising thetemperature of the LFFC to 50° C., and keeping 1M MeOH as fuel, anoverall increase in performance was observed as expected (see FIG. 11).However, when 8M MeOH was used again as fuel the improvements weresmaller suggesting that at elevated temperatures transport issues to theanode are less of an issue and that the cathode is most likely limitingthe LFFC under these conditions. When the commercially available DMFCwith 1 M MeOH was examined, a slightly better performance was observed,than the LFFC under the same conditions. However, when the DMFC wasexposed to 8 M MeOH the performance was negatively impacted as a resultof crossover. This study illustrated was that the LFFC design has alower cell resistance, better mass transport characteristics and a muchlower crossover rate than a traditional DMFC design.

1. An electrochemical cell, comprising: a first electrode, a secondelectrode, a porous separator, between the first and second electrodes,a first channel, having an inlet and an outlet, and a second channel,having an inlet and an outlet, wherein the first channel is contiguouswith the first electrode and the porous separator, and the secondchannel is contiguous with the second electrode and the porousseparator.
 2. The electrochemical cell of claim 1, such that when afirst liquid flows through the first channel laminar flow isestablished, and when a second liquid flows through the second channellaminar flow is established.
 3. The electrochemical cell of claim 1,wherein a distance between the first electrode and the second electrodeis at most 1 cm.
 4. The electrochemical cell of claim 1, wherein adistance between the first electrode and the second electrode is at most1 mm.
 5. The electrochemical cell of claim 1, wherein a distance betweenthe first electrode and the second electrode is at most 0.5 mm.
 6. Theelectrochemical cell of claim 1, wherein the electrochemical cell is afuel cell.
 7. The electrochemical cell of claim 1, wherein the firstelectrode is a gas diffusion electrode.
 8. The electrochemical cell ofclaim 4, wherein the first electrode is a gas diffusion electrode. 9.The electrochemical cell of claim 1, wherein the porous separator has athickness of 1 to 100 microns.
 10. The electrochemical cell of claim 1,wherein the porous separator has a pore size of 5 nm to 5 microns. 11.The electrochemical cell of claim 1, wherein the porous separator has apore density of 10⁶ to 10¹¹ pores/cm².
 12. The electrochemical cell ofclaim 1, wherein the porous separator has a porosity of 0.1 to 50%. 13.The electrochemical cell of claim 6, wherein the first electrode is agas diffusion electrode.
 14. The electrochemical cell of claim 13,wherein a distance between the first electrode and the second electrodeis at most 1 cm.
 15. The electrochemical cell of claim 14, wherein theporous separator has a thickness of 1 to 100 microns.
 16. Theelectrochemical cell of claim 14, wherein the porous separator has apore size of 5 nm to 5 microns.
 17. The electrochemical cell of claim14, wherein the porous separator has a pore density of 10⁶ to 10¹¹pores/cm².
 18. The electrochemical cell of claim 14, wherein the porousseparator has a porosity of 0.1 to 50%.
 19. The electrochemical cell ofclaim 18, wherein the porous separator has a thickness of 1 to 100microns, a pore size of 5 nm to 5 microns, and a pore density of 10⁶ to10¹¹ pores/cm².
 20. A method of generating electricity, comprising:flowing a first liquid through a first channel; and flowing a secondliquid through a second channel; wherein the first channel is contiguouswith a first electrode and a porous separator, the second channel iscontiguous with a second electrode and the porous separator the firstliquid is in contact with the first electrode and the porous separator,the second liquid is in contact with the second electrode and the porousseparator, and complementary half cell reactions take place at the firstand second electrodes. 21-38. (canceled)
 39. An electrochemical cell,comprising: a first electrode, a second electrode, a first channel,contiguous with the first and second electrodes, wherein the firstelectrode is a gas diffusion electrode, such that when a first liquidflows through the channel in contact with the first electrode and asecond liquid flows through the channel in contact with the secondelectrode, laminar flow is established in both the first and secondliquids. 40-53. (canceled)